High-density, high-speed, non-volatile, low-power, and low-cost are common goals shared by many memory devices. But all these goals cannot be realistically obtained in practice, and some trade-offs are inevitable. The particular applications dictate which compromises are to be made. For example, static random access memory (SRAM) is fast, but usually comes at the cost of lower density. Such a memory is useful in CPU-cache memory applications. Dynamic random access memory (DRAM) is high density, but is not non-volatile. So DRAM is usually used in main memory applications for general purpose computers.
Newer memory types like magnetic random access memory (MRAM) are inherently non-volatile, but still have to find compromises between density, access speed, etc. Three types of MRAM have been developed based on different magnetic phenomenon, e.g., anisotropic, giant, and tunneling magneto-resistance.
The tunneling magneto-resistance type of MRAM is of interest here. A cross-point array of magnetic tunneling junction (MTJ, sometimes also called spin-dependent tunneling junction, or SDT junction) memory cells allows direct addressing. Each cell appears as a resistance that depends on the digital data value being stored.
The conventional MTJ memory cell comprises two magnetic layers separated by an electrical insulator. The insulator is so thin that it is subject to tunneling currents between the magnetic layers it contacts. Such tunnel currents appear as an electrical resistance that depends on the relative orientation of the magnetizations of the two magnetic layers. The upper and lower magnetic layers are deposited as ellipsoids so that their magnetizations will occur in one of two preferred directions, e.g., longitudinal with the ellipsoid. Other shapes, such as rectangular or asymmetric, and of appropriate aspect ratios (ie, length-to-width ratios) may also be used.
The lower magnetic layer is fabricated with a high coercivity material and is permanently magnetized in a set direction during an annealing process step. This layer serves as the reference layer. The upper magnetic layer comprises a lower coercivity material whose magnetization direction is switched by column and row data-write currents that produce write fields that combine at the targeted cross-point array intersection. This layer serves as the data or storage layer (sometimes also called the bit layer or the sense layer). In other versions, the data and the reference layers may be deposited in the opposite order. In a version of the memory cell called the “spin-valve”, the reference layer is “pinned” by exchange coupling by an adjacent antiferromagnetic layer. In such a spin-valve, the orientation of the magnetization of the pinned reference layer remains substantially fixed.
The electrical resistance through the tunnel barrier is dependent on the relative orientations of the magnetizations in the data and reference layers. When these magnetizations are oriented in the same direction, the electrical resistance will have a certain value and when the magnetizations oppose each other the resistance will be changed. This change in resistance is the tunneling magneto-resistance (TMR) effect and the state of the data layer can be read by measuring the apparent electric resistance across the layers. Typically, the MTJ resistance is low when the magnetization orientations are parallel, and high when antiparallel.
As cells become smaller, thermal stability issues become more important. The coercivity, or magnetic switching field, of small magnetic memory cells must be large enough to ensure that stored information is not lost because of random switching induced by environmental influences. The coercivity required to produce a thermally stable memory cell increases as the memory cell is made smaller. Unfortunately, the necessity to generate the larger field strength makes switching of the smaller memory cells during the write operation more difficult.
It is known that increasing the temperature of the memory cell lowers the magnetic field strength that is required for switching. This is because the magnetic material now has a higher thermal energy at this increased temperature. Further, when an electrical current passes through the memory cells, heat is developed in the cells. However, the developed heat is easily conducted through the bit and word lines away from the memory cell and therefore cannot be utilized to facilitate switching of the magnetic memory cell.
There is therefore a need for a magnetic memory device in which loss of heat from the magnetic memory cell is reduced and therefore the heat can be utilized to ease cell state switching.